In the past, copper sheets have been used in various industrial fields as a material for connectors, electrodes, connecting terminals, terminals, sensing members, heat sinks, bus bars, backing plates, molds and motor members such as end rings and rotor bars by utilizing excellent electrical and heat conductivity thereof. However, since pure copper including C1100 and C1020 has low strength, the use per unit area is increased to ensure the strength and thus cost increases occur and weight increases also occur.
Cr—Zr copper (1 mass % Cr-0.1 mass % Zr—Cu), which is a solution aging⋅precipitation type alloy, is known as a high-strength and high-electrical conductivity copper alloy. However, in general, this alloy is prepared through a heat treatment in which a hot-rolled material is re-heated at 950° C. (930° C. to 990° C.) and then subjected to immediate quenching and aging. Alternatively, the alloy is prepared through a series of heat treatments in which after hot rolling, a hot-rolled material is further subjected to plastic forming by hot or cold forging or the like in some cases, subjected to a solution heat treatment so as to be heated at 950° C. and rapidly cooled, and then subjected to aging. The high-temperature process of 950° C. not only requires significant energy, but oxidation loss occurs when the heating operation is performed in the air. In addition, because of the high temperature, diffusion easily occurs and the materials stick to each other, so an acid cleaning process is required.
For this reason, the heat treatment is performed at 950° C. in an inert gas or in vacuum. However, although the oxidation loss is prevented, the cost is increased, extra energy is also required and the sticking problem is not solved. Further, regarding the characteristics, grains become coarse and problems occur in fatigue strength since the heating operation is performed at high temperatures. Meanwhile, in a hot rolling process in which the solution heat treatment is not performed, only very poor strength can be obtained. In a hot rolling process, in the case of Cr—Zr copper, coarse grains are precipitated during the hot rolling due to a decrease in material temperature during the hot rolling, and thus a sufficient solution heat-treated state cannot be obtained even when a quenching operation is immediately performed after the hot rolling. In addition, Cr—Zr copper requires special management since a temperature condition range of the solution heat-treating is narrow, and if a cooling rate is not high enough, the solution is not realized. Moreover, since a large amount of active Zr and Cr is included, restrictions are imposed on the melting and casting. As a result, excellent tension strength and electrical conductivity are obtained, but the cost is increased.
In the automobile field using a copper sheet, while a decrease in the vehicle body weight is required to improve fuel efficiency, the number of components such as a connecting terminal, connector, relay and bus bar is increased due to the high-level informatization and the acquisition of electronic properties and hybrid properties (an increase in the number of electrical components) in a vehicle, and the number of members such as a heat sink for cooling the mounted electronic components is also increased. Accordingly, a copper sheet to be used is required to have a smaller thickness and higher strength. Originally, in comparison to the case of home appliances and the like, the usage environment is harsh, as the temperature of the vehicle interior, as well as the engine room, increases in summer (especially). Further, due to a high-voltage and high-current usage environment, it is particularly required to lower stress relaxation properties when a copper sheet is used in a connecting terminal, a connector and the like. The low stress relaxation properties mean that a contact pressure or spring properties of a connector and the like are not lowered in a usage environment of, for example, 100° C. In this specification, in a stress relaxation test to be described later, a low stress relaxation rate indicates “low” or “good” stress relaxation properties and a high stress relaxation rate indicates “high” or “bad” stress relaxation properties. It is preferable that a copper alloy rolled sheet has a low stress relaxation rate.
In addition, in many cases, due to the demands for high reliability, important electrical components are joined to each other by brazing, not soldering. Further, for example, also in motors, brazing is employed to join an end ring and a rotor bar, and high material strength is required after the joining to improve the performance speed of motors. Examples of a brazing filler material include Bag-7 (56Ag-22Cu-17Zn-5Sn alloy brazing filler material), described in JIS Z 3261, and a recommended brazing temperature thereof is in the high temperature range of 650° C. to 750° C. Accordingly, a copper sheet for use in relays, connecting terminals, sensing members, rotor bars, end rings and the like is required to have heat resistance of, for example, about 700° C.
In addition, for backing plates, molds and the like, non-deforming with respect to a temperature increase during manufacturing or use is required. For example, a material is required which has high strength at high temperatures of 300° C. to 400° C. Moreover, in some cases, friction diffusion welding is employed to join sheets to each other during manufacturing and thermal spraying is carried out in a process for increasing the heat resistance of a surface. It is required that a decrease in strength and electrical conductivity is small even upon exposure to high temperatures in a short time. In addition, for power modules and the like, copper for use in a heat sink or a heat spreader is joined to ceramic as a base sheet. Soldering is employed for the above joining, but Pb-free has become general for solder as well, and thus high-melting point solder such as Sn—Cu—Ag is used. In mounting a heat sink, a heat spreader and the like, it is required that not only does softening not occur but also that deformation and warpage do not occur and a thinner wall thickness is demanded in view of weight reduction and economy. A copper material is required to be not easily deformed even when exposed to high temperatures. That is, a copper material is required to have high heat resistance and high strength at high temperatures.
A copper alloy is known which includes 0.01 to 1.0 mass % of Co, 0.005 to 0.5 mass % of P and the balance including Cu and inevitable impurities (for example, see JP-A-10-168532). However, such copper alloy is also insufficient in both strength and electrical conductivity.